1. Field of the Invention
The present invention relates to an exhaust gas treatment system.
2. Description of the Related Art
NOx, SOx, carbon monoxide (CO), and volatile organic compounds (VOCs) such as saturated hydrocarbons except methane and ethane and unsaturated hydrocarbons e.g. ethylene are contained in exhaust gases discharged from industrial devices of various types (e.g. automobile engines, air craft gas turbines, and gas turbines for power generation), chemical plants, factories of various types, and so on. Such components contained in exhaust gases have been treated by adopting a system using catalysts for the removal of CO, VOCs and the like in combination with a denitration catalyst.
The current system requires the catalysts for the removal of CO, VOCs and the like at an upstream stage with a higher temperature because the catalysts have to be exposed to a temperature higher than 400° C. for improving the conversion rates of CO and VOC (see e.g., Japanese Patent Provisional Publication No. 10-309462). However, the system operation under the high temperature presents problems in terms of the durability of the apparatus such as the deterioration of the catalysts for the removal of CO, VOCs and the like.
Among VOCs, particularly, saturated hydrocarbons (NMHCs) such as propane shows a low burning velocity in the presence of the catalysts for the removal of CO, VOCs and the like conventionally placed at the upstream stage. Most of the saturated hydrocarbons passed through the upstream stage remaining unreacted unless the temperature is increased to 550° C. which is the upper limit of the suitable temperature range described in Japanese Patent Provisional Publication No. 10-309462. As a result, the saturated hydrocarbons unreacted are partially oxidized with the denitration catalyst placed downstream at 400° C. or lower and become a source of carbon monoxide at the final outlet of the system.
An object of the present invention is to provide an exhaust gas treatment system, which allows for VOC removal at lower temperatures and thereby improves the durability of catalysts and suppresses carbon monoxide generation at the final outlet of the system.